Thermal insulation structures comprising layers of expanded graphite particles compressed to different densities and thermal insulation elements made from these structures

ABSTRACT

Thermal insulation structure having at least one flexible layer based on compressed expanded graphite particles characterised in that the density of the said flexible layer is equal to at least 0.4 g/cm3 (400 kg/m 3 ) and in that the thermal insulation structure also includes another layer close to the flexible layer based on compressed graphite particles with a lower density, typically less than 0.4 g/cm 3  (400 kg/m 3 ). Preferably, the dense compressed expanded graphite layer has a density of between 0.5 and 1.6 g/cm 3  (500 and 1600 kg/m 3 ) and the sub-dense compressed expanded graphite layer has a density of between 0.05 and 0.3 g/cm 3  (50 and 300 kg/m 3 ). Thermal insulation elements are also described that are designed to be fitted on furnaces operating under non-oxidising atmosphere and at temperatures of more than 800° C.

TECHNICAL FIELD

This invention relates to the manufacture of refractory carbonaceousmaterials that can be used as high temperature thermal insulation infurnaces operating at high temperatures and under a non-oxidisingatmosphere.

BACKGROUND OF THE INVENTION

Equipment operating at high temperature in a wide variety of domainssuch as the manufacture of electronic components, optical fibres,reactor blades, etc. uses different forms of carbon-based thermalinsulation. The reasons for this choice are:

-   -   the refractory nature of carbon, which is solid up to 3000° C.;    -   the low thermal conductivity of low density carbon base        materials;    -   a relatively low production cost;    -   the possibility of obtaining very pure carbon and consequently        limiting possible contamination of elements being worked at high        temperature, which is an essential aspect for processes related        to the electronic industry.

The most frequently used forms of carbon-based thermal insulation are:

-   -   bricks based on carbon powder bonded together by a binder        derived from carbonation of a liquid rich in carbonaceous        material (e.g. pitch, phenolic resin, etc.). These bricks are        very economic and are widely used in metallurgy (blast furnaces,        furnaces with atmosphere saturated in CO);    -   so-called flexible carbon fibre felts made from carbon and/or        graphite fibres formed into low-density felts, typically with a        density of 0.1 g/cm3 (100 kg/m3);    -   so-called rigid carbon fibre felts, made from carbon and/or        graphite fibres, bonded together by binders derived from        carbonation of a liquid rich in carbonaceous material, or by        deposition of a pyrocarbon in the gaseous phase onto a fibrous        preform to be consolidated. The densities of these rigid felts        are between 0.1 g/cm3 (100 kg/m3) and 0.3 g/cm3 (300 kg/m3);    -   carbon black, contained in a chamber to form a compacted powder        wall between the hot area to be insulated and a cold        environment. Densities of compacted carbon black typically vary        between 0.05 g/cm3 (50 kg/m3) and 0.2 g/cm³ (200 kg/m³);    -   finally, the last family consists of materials described in        particular in patent U.S. Pat. No. 3,404,061, and comprises        particles of expanded graphite compressed in the absence of a        carbonaceous binder to obtain solid structures with densities        typically between 5 and 137 pounds per cubic foot, in other        words between about 80 kg/m³ and 2300 kg/m³. There are several        means of obtaining expanded graphite particles. They are        described for example in U.S. Pat. No. 3,404,061 (milling,        attack of spaces between hexagonal reticular planes by oxidising        or halogenated agent water impregnation, heating to a        temperature of more than 100° C.) or in U.S. Pat. No. 5,582,781        (milling, immersion in liquid nitrogen then thermal shock). In        general, expanded natural graphite is used. When the compression        applied to expanded graphite particles results in a density of        more than about 0.4 g/cm³ (400 kg/m³), flexible graphite strips        having a good mechanical strength are obtained.

Each type of thermal insulation mentioned above has advantages anddrawbacks which make its use more or less suitable for the special needsof each process.

The invention relates particularly to thermal insulation structuresbased on compressed expanded graphite particles. Presently, thesestructures are not very widely used, in comparison with structures basedon carbon fibres. They are two types of reasons against a broaddistribution of these products, despite their very competitive thermalproperties:

-   1) structures based on compressed expanded graphite particles are    very fragile if their density is less than about 0.2 g/cm³ (200    kg/m³), structures with densities lower than this value are    extremely fragile and are practically impossible to use;-   2) the most natural solution to overcome this problem of extreme    fragility is to increase the densities obtained after compression,    but the result is then the loss of insulation performance;-   3) techniques for manufacturing this type of structure are not very    productive.

They generally involve the following steps:

-   -   an extremely lightweight expanded graphite powder is produced,        typically with a density of 0.003 g/cm³ (3 kg/m³) to 0.005 g/cm³        (5 kg/m³);    -   this powder is placed in a compression chamber with an        appropriate geometry to achieve the required shapes;    -   the powder is compressed until a solid with the required density        is obtained.    -   The ratio between the density of the initial powder and the        density of the finished product makes it necessary to stack        powder to a height at least 40 times more than the thickness of        the required insulation product. Thus, if the objective is to        obtain a 10 mm-thick thermal insulation product (typical value        for carbon fibre felts), a mould with a minimum height of 400 mm        has to be filled and slow compression has to be applied over a        distance of at least 390 mm. Therefore, the method is not very        productive and quality defects are easily produced, caused by        difficulties in “degassing” the powder during the compression        phase and by material heterogeneity.

-   4) due to their fragility, structures based on expanded graphite    particles that have been compressed with a low compression ratio    have the defect that they release graphite particles that are not    well bonded to the material mass. This causes undesirable pollution    in the chambers to be insulated and this pollution is a particularly    severe handicap in industries such as those dedicated to the    manufacture of semiconductors, in which cleanliness is of overriding    importance.

In order to obtain thermal insulation materials which are particularlysuitable for high temperature furnaces operating in a non-oxidisingatmosphere, some patents disclose the use of multilayer sheetscomprising at least one flexible layer made of a material based oncompressed expanded graphite particles.

Thus, U.S. Pat. No. 4,279,952 describes a composite structure containingtwo of the forms of carbonaceous insulation mentioned above; oneflexible carbon fibre felt layer (density between 60 and 100 kg/m³) isbonded to at least one flexible sheet made of a material based oncompressed expanded graphite particles with a density of between 600 and1600 kg/m³. The bond between the two layers is made by a carbon-basedbinder, for example a carbonisable polymeric resin. However, thiscomposite structure has disappointing thermal insulation propertiesconsidering its relatively high cost, and the risk of pollution of thechamber is not fully eliminated since flexible felts are sources oflarge amounts of dust. In particular, fibrils originating from thevisible edges of the felts become detached, and they are very easilycarried in the air because they are so small.

Another structure is proposed by U.S. Pat. No. 4,888,242. In this patentthe layers in the multilayer are not intimately bonded to each otherover their entire surfaces since some layers (made of materials based oncompressed expanded graphite particles) are corrugated before beingconnected to other layers that remain flat; contact surface areasbetween layers are small which improves the thermal insulationproperties of the multilayer thus formed. However, this type ofstructure is difficult to make. If it is to contain a small number oflayers, then large amplitude corrugations are necessary which arefirstly difficult to produce, and secondly weaken the structure due tocompression forces applied perpendicular to the flat surfaces. Inpractice, small amplitude corrugations are necessary, therefore therewill be a large number of corrugated layers to be stacked in thestructure, which implies a lot of gluing. The structure finally obtainedhas a fairly high average density and fairly disappointing thermalinsulation properties, considering its high manufacturing cost.

Finally, U.S. Pat. No. 6,413,601 describes a thermal insulation furnacejacket obtained by using a flexible strip made of a material based oncompressed expanded graphite particles, the said strip being woundspirally in several layers. The layers are bonded to each other by abonding material, typically a phenolic resin. Resin degassing problemsduring carbonation are avoided by inserting a material, which decomposesunder the effect of heat between two layers of the spiral which arecoated with phenolic resin. Typically, a paper sheet is used, and as thepaper decomposes it creates diffusion paths through which gases derivedfrom carbonation of the resin will escape.

The applicant has attempted to make under satisfactory economicconditions a thermal insulation structure that does not have thedisadvantages mentioned above and that can be used in the manufacture ofthermal insulation elements, particularly insulation for furnacesoperating at high temperatures and in non-oxidising atmosphere.

SUMMARY OF THE INVENTION

A first purpose of the invention is a thermal insulation structurecomprising at least one flexible layer based on compressed expandedgraphite particles characterised in that the density of this layer isequal to at least 0.4 g/cm³ (400 kg/m³) and in that the said thermalinsulation structure also comprises another layer based on compressedgraphite particles with a lower density, typically less than 0.4 g/cm³(400 kg/m³).

The two layers are preferably adjacent and are bonded to each other by abinder obtained by carbonation of a binding agent typically a liquidrich in carbonaceous material, e.g. a phenolic resin, a furfuryl resin,a pitch, etc. These layers may have discontinuous contact surfaces as inU.S. Pat. No. 4,888,242 but preferably the two adjacent layers areintimately bonded together over their entire contact surface by thecarbonaceous binder. For conciseness, we will call the first layer the“dense expanded graphite layer” and the second layer the “sub-denseexpanded graphite layer” In the remainder of this description.

Thus, the present invention makes it possible to obtain thermalinsulation structures made by a combination of layers based on expandedgraphite particles compressed to different densities and bondedtogether, this solution avoiding the problems mentioned above and evenmaking it possible to benefit from the excellent thermal insulationproperties of expanded graphite when it is used at very low density.

The structure according to the invention benefits from the complementaryproperties of the two layers. The layer based on compressed graphiteparticles with a higher density (the dense expanded graphite layer) hasgood mechanical strength and this property is also conferred to theresulting structure. The layer based on compressed graphite particleswith a lower density (the sub-dense graphite layer) is significantlymore fragile than the first layer but it has better thermal insulationproperties, so that the resulting structure can be used for themanufacture of thermal insulation elements such as jackets for hightemperature furnaces. Moreover, since it is porous, it enables diffusionof gases emitted during the phase in which the carbonaceous binder thatbinds the two layers is being carbonised, which avoids the formation ofdefects in the said bonding layer.

Joining and gluing two dense layers to each other introduces degassingproblems and special solutions have to be adopted such as thosedescribed in U.S. Pat. No. 6,413,601. Thus, the combination of anon-dense material and a dense material enables gases emitted during theheat treatment phase originating from decomposition of the bondingmaterial to be easily evacuated by diffusion through the non-densematerial that has retained a very high permeability to gases. Therefore,according to this invention, it is important to maintain the presence ofa sub-dense layer of expanded graphite close to the binder obtained bycarbonation.

The structure according to the invention has at least one alternation ofadjacent dense and sub-dense layers with different physical propertiesand different thicknesses; the layer based on compressed graphiteparticles having a higher density (the dense expanded graphite layer)may also be as thin as possible, but sufficiently dense to give theglobal structure the required flexibility and mechanical strength. Thelayer based on compressed graphite particles with a lower density (thesub-dense expanded graphite layer) may also be as thick as possible toincrease the global thermal insulation of the structure.

A typical two-layer structure according to the invention comprises thefollowing two adjacent layers:

-   a) a “thick” layer preferably less than 40 mm thick, typically    between 5 and 20 mm, which is not very dense, by limiting the    density obtained by compression of graphite particles to small    values of the order of 0.1 g/cm³ (100 kg/m³) and typically within    the range of 0.05 g/cm³ (50 kg/m³) to 0.30 g/cm³ (300 kg/m³), the    said layer being unsuitable for use as thermal insulation by itself    due to its excessive fragility;-   b) a “thin” layer, preferably with a thickness of less than 2 mm,    and typically between 0.15 and 1.5 mm thick, with a density    typically within the 0.5 to 1.6 g/cm³ (500 to 1600 kg/m³) range.

The elementary structure composed of two adjacent layers made ofexpanded graphite compressed to different densities and bonded to eachother by means of a carbonaceous binder may be used directly for makinginsulation elements for high temperature furnaces. In this case, anoriented structure will preferably be used such that its face occupiedby the dense layer of expanded graphite surrounds the furnace chamberprotecting the latter from possible emission of particles originatingfrom the sub-dense layer.

This type of elementary structure can also be stacked on itself severaltimes so as to obtain a structure with alternating dense and sub-denselayers which can give a consistent and thick thermal insulationassembly. In this case, the elementary thicknesses of dense andsub-dense expanded graphite layers are different but the totalthicknesses of each are preferably in accordance with the values of thetypical two-layer structure provided above.

Preferably, to avoid possible pollution by emission of particlesoriginating from the sub-dense layer with a low mechanical strength, thethermal insulation element obtained by stacking the elementary structureaccording to the invention also has all its outside faces covered by alayer of expanded graphite compressed to a density of more than 0.4g/cm³ (400 kg/m³), and preferably between 0.5 and 1.6 g/cm³ (500 to 1600kg/m³).

The thermal insulation structure according to the invention can be usedto make insulating elements with different shapes using various methods:

-   -   a thick multilayer strip is made by        lamination/gluing/carbonation of the binding agent. Elementary        bricks are made by cuffing them out of the strip. Shapes adapted        to the application concerned can also be cut out directly;    -   at least two thin two-layer strips are made by lamination and        gluing. Two strips are assembled by co-lamination, in which one        face of at least one strip consists of a sub-dense layer and the        interface is glued using a carbonisable liquid binder. This        operation is repeated as many times as necessary to obtain a        multilayer strip with the required thickness. The binder is        carbonised, and the elementary bricks are then cut out or they        are cut to shape, as in the previous example;    -   a multilayer strip is made by lamination/gluing/carbonation of        the binding agent and a cylindrical jacket is then made by        winding the said strip spirally on one or more layers, after        gluing at least one of the faces with a binding agent,        preferably a face occupied by a sub-dense layer. The cylindrical        jacket is obtained by winding the number of layers necessary to        obtain the required thickness in spiral form. The strip is        sufficiently flexible to accept the imposed bending without        damage during winding. If the sub-dense layer is thin enough, it        can be wound spirally with the sub-dense layer on the outside        face (the convex side), even if the sub-dense layer is not very        resistant to tension stresses. For example, a 200 mm diameter        cylindrical jacket was made by winding a two-layer structure        consisting of a 5 mm thick sub-dense layer with a density of        0.15 g/cm³ glued onto an 0.5 mm thick dense layer with a density        of 1 g/cm³. If the sub-dense layer is thicker or even less        dense, the dense layer that has better resistance to tension        stresses generated by binding when winding, is preferably placed        towards the outside of the cylinder (the convex side).        Regardless of the position of the sub-dense layer (on the        concave or the convex side), the cylindrical face of the        cylindrical jacket obtained that is occupied by the sub-dense        layer is preferably covered, for example, with a flexible strip        of dense compressed expanded graphite, itself, if necessary,        glued with a binding agent. Once the winding has been completed        and the glued dense flexible strip has been deposited on the        face occupied by the sub-dense layer, the jacket is heat treated        to carbonise the binding agent located between the wound layers.        This is usually done by applying a heat treatment under a        non-oxidising atmosphere at a temperature equal to at least the        temperatures that the thermal insulation will need to resist        during use (typically 800° C., and preferably 1000° C. or more);    -   a non-dense strip is chosen with a sufficient density and/or a        sufficiently thin strip to be able to resist shaping by bending        without damage, glue is then applied to at least one face, which        is afterwards covered by a thin dense layer and a thermal        carbonation treatment is finally carried out on the binding        agent;    -   for complex shapes, moulding to the required shape is done using        expanded graphite that is slightly compressed to obtain the        sub-dense quality with the required thermal insulation        properties and the surface of the moulded part is then glued,        covered with a thin dense layer and a carbonation heat treatment        is carried out on the binding agent. In the latter case, the        thickness of the sub-dense expanded graphite layer is not        necessarily uniform.

Regardless of the production process used, the thermal insulationelement made using an insulating structure according to the inventionhas innovative and attractive properties. Example 1 illustrates aninsulating element obtained from a structure according to the inventionand presents the advantages obtained compared with insulating elementsaccording to the prior art. The invention also covers a thermalinsulation element of this type forming part of the wall of the chamberof a furnace operating at high temperatures, typically more than 800° C.and in a non-oxidising atmosphere. It may be in the form of bricks, suchthat the assembly of several of these bricks forms the surface of thefurnace chamber, or in the form of a cylindrical wall in one or moreparts making up the furnace combustion chamber.

Another purpose of the invention is a process for manufacturing the saidstructure according to the invention.

The first step is producing at least two components based on compressedgraphite particles according to the known prior art, such as the processdescribed in U.S. Pat. No. 3,404,061 (milling, attack of the spacesbetween hexagonal reticular planes by an oxidising or halogenated agent,impregnation with water, heating to a temperature of more than 100° C.)or the process described in U.S. Pat. No. 5,582,781 (milling, immersionin liquid nitrogen then thermal shock).

A “thick” and not very dense strip is made with a thickness of less than40 mm, typically between 5 and 20 mm thick, by limiting the densityobtained by compression to low values (of the order of 0.1 g/cm³ (100kg/m³) and typically within the range 0.05 g/cm³ (50 kg/m³) to 0.30g/cm³ (300 kg/m³). A “thin”, dense, strip with a thickness of less than2 mm, and typically between 0.15 and 1.5 mm, and a density typicallywithin the range 0.5 to 1.6 g/cm³ (500 to 1600 kg/m³), is also made.These two products can be made in the form of a continuous strip, usingtypical equipment used to make flexible graphite sheets (rolling lines),by taking the strip either from the part of the upstream side of therolling lines (used for shaping flexible graphite sheets) to obtain the“thick” sub dense layer, or in the part on the downstream side of thesaid rolling lines to obtain the dense thin layer. The process is fastand economic, but it is only suitable for production of continuousstrips with fixed width (typically 1 m or 1.50 m wide), which then haveto be cut to obtain complex shapes if required.

Obviously, the mould/compression filling technique could also be used tomake these products and to obtain the required density, but this processis more expensive in terms of cycle time and labour time, and is alsomore flexible with regard to the shapes obtained.

The two layers with different natures are then joined or assembled bygluing to form sandwich or multilayer structures, which will comprisealternating non-dense thick layers and dense thin layers, with a minimumof at least two elements.

The gluing technique typically consists of coating the non-dense thicklayer with a carbon rich liquid solution, e.g. a phenolic resin, afurfuryl resin, pitch, etc. Almost all solvents in the solution, if any,are then eliminated by slow drying. The next step is to apply a densethin layer on the coated surface followed by a heat treatment processunder a non-oxidising atmosphere and at a temperature equal to at leastthe temperatures that the thermal insulation will have to resist duringuse (typically 800° C. and preferably 1000° C. or more).

One variant embodiment of manufacturing an insulating element with astructure according to the invention is making a thin non-dense layerthat is afterwards consolidated by a dense layer added to it theapplicant has demonstrated that it is possible to bend thick non-denselayers before gluing for strip thicknesses of up to 25 mm. Afterbending, these strips placed in the shape of an arc are glued and densereinforcing layers are applied onto one or both faces. The assembly isthen heat treated while being held in (bent) shape by a graphiteconforming jig surrounding the product. The structure at the output fromthe heat treatment is curved, geometrically stable in the form of anarc. A combination of these arcs can give circular cylindrical thermalinsulation assemblies, and this shape is used on many furnaces operatingunder a vacuum and at high temperatures.

In another variant, flexible thin multilayer sheets are produced,composed of alternating dense expanded graphite layers and non-denseexpanded graphite layers. It has been demonstrated that one of thesesheets can be wound on itself around a spiral without using the bendingtechnique. The assembly is heat treated in an appropriate assemblyholding the spiral in place. The structure at the output from thetreatment is a geometrically stable cylindrical structure composed of acontinuous sheet that can be used as circular thermal insulation elementfor furnaces. Preferably, the flexible strip used is a dense/sub-densetwo-layer or a sub-dense/dense/sub-dense three-layer material, such thatgases emitted during the final carbonation treatment of the bindingagent can diffuse without generating any defects. Preferably, itsinternal face (the bore) and external face are covered with a thin layerof dense expanded graphite, to provide the cylindrical structure thusmade with good mechanical strength and to prevent unwanted pollution ofthe furnace.

EXAMPLE

The applicant made an elementary brick with a structure consisting of a10 mm thick non-dense layer (0.1 g/cm³ (100 kg/m³)) sandwiched betweentwo dense layers (1 g/cm³ (1000 kg/m³)) each 0.5 mm thick the completeassembly being made in the form of a board with dimensions 500 mm×500mm×11 mm.

A “thick”, non-dense, 10 mm thick strip is obtained by limiting thedensity by compression to 0.1 g/cm³ (100 kg/m³). Two “thin”, dense, 0.5mm thick strips are obtained by compressing the expanded graphiteparticles until a density of 1 g/cm³ (1000 kg/m³) is obtained.

These strips are obtained using a conventional installation for thecontinuous production of flexible graphite sheets called “Papyex”(registered trademark) and made by the applicant. This conventionalinstallation comprises rolling lines capable of making strips up to 1.5m wide. The thick strip is taken from the upstream side of the rollinglines and the thin strips are taken from the downstream side.

The limitation to the thickness and density required for the non-denseelement makes it possible to use these conventional installations thatare capable of producing strips with weight per unit area typicallybetween 500 and 3000 g/m² (the thicknesses of which will vary during thevarious rolling steps), in other words that are capable of continuouslyproducing strips with a density of 0.1 g/cm³ (100 kg/m³) and thicknessesof up to 30 mm.

This type of continuous production is capable of very significantlyreducing costs compared with moulding/compression processes, the costratio for a given volume of thermal insulation produced being of theorder of 2.5 or more, with the continuous solution being less expensive.Finally, it is important to mention the advantage of the continuousprocess for stability of properties of the products obtained, which issignificantly easier to guarantee than with production by moulding ofindividual parts.

Once these strips have been made and collected, the next step is toco-laminate a thick strip sandwiched between two thin strips, eachinterface being previously coated with a phenolic resin, which is acarbonisable liquid binding agent. The assembly is then heated up to atemperature close to 1000° C. which causes carbonation of the bindingagent. The bricks are then cut to the required dimensions.

This type of brick has the following advantageous properties:

-   1) emission of particles through the faces of the structure: this is    typical of particle emissions through flexible graphite sheets. The    compressed expanded graphite sheets emit almost no particles when    the density is greater than 1 g/cm³ (flexible graphite sheets). They    emit a few fairly large particles when the density is less than 0.2    g/cm³. Compared with structures including carbon fibre felts, like    those described in U.S. Pat. No. 4,279,952, structures comprising    low density compressed expanded graphite do not represent unwanted    dust sources to such an extent, since the number of emitted    particles is small and their large dimensions do not facilitate    dispersion; they “drop” rather than “fly”;-   2) mechanical bending strength resisting a load applied    perpendicular to the main faces: ultimate strength 3 MPa, to be    compared with 0.7 MPa for structures based on expanded graphite    particles compressed to 0.2 g/cm³ (200 kg/m³) and not bonded to    dense layers;-   3) thermal conductivity at low temperature measured by resistance to    the passage of a thermal flux perpendicular to the main faces: with    a hot face kept at 200° C.: 0.35 W/m.K to be compared with 0.6 W/m.K    measured under the same conditions on a structure based on expanded    graphite particles compressed to 0.2 g/cm³ (200 kg/m³) (advantage of    the low density in the case of the composite structure);-   4) low chemical reactivity of the sandwich, for example in    comparison with carbon felt, due to the use of a layer of dense    compressed expanded graphite with very low permeability to gases and    a very good chemical inertia, on the faces. This final point    demonstrates the advantage of this type of structure in comparison,    for example, with the structure described in U.S. Pat. No.    4,279,952, in which the composite structure contains a carbon fibre    felt that is chemically more reactive than the compressed expanded    graphite.

ADVANTAGES

-   -   Rigid thermal insulation structures according to the invention        can be made from an abundant and inexpensive raw material        (complexed natural graphite that is obtained in the phase        preceding the expansion step resulting in expanded natural        graphite). Other types of rigid insulation based mainly on        carbon fibres start from a significantly more noble raw        material, five to ten times more expensive. If shaping costs are        included, there is no exaggeration in saying that thermal        insulation according to the invention is about 30% less        expensive to make than more conventional thermal insulation        systems for the same thermal insulation function, except carbon        black based systems but, for those, there are other severe        problems with systems made from carbon black that might even        make them unacceptable, and particularly cleanliness properties.

1-18. (canceled)
 19. Thermal insulation multi-layer structure comprisingat least one flexible layer based on compressed expanded graphiteparticles characterised in that the density of the said flexile layer,called dense compressed expanded graphite layer, is between 0.5 and 1.6g/cm³ (500 and 1600 kg/m³) and in that the said thermal insulationstructure also comprises another layer called sub-dense compressedexpanded graphite layer, based on compressed graphite particles with alower density, which is between 0.05 and 0.3 g/cm³ (50 and 300 kg/m³),said dense and sub-dense layers being adjacent and bonded to each other.20. Thermal insulation structure according to claim 19 in which the saiddense and sub-dense layers made of compressed expanded graphite areadjacent and are bonded to each other by carbonation of a carbonisablebinding agent, typically phenolic resin, furfuryl resin or pitch. 21.Thermal insulation structure according to claim 20 in which the adjacentdense and sub-dense layers made of compressed expanded graphite areintimately bonded together over their entire contact surface. 22.Thermal insulation structure according to claim 19 obtained by stackingthe said adjacent dense and sub-dense layers, with one alternation ofdense and sub-dense layers made of compressed expanded graphite. 23.Thermal insulation structure according to claim 19 in which the saidsub-dense layer or layers made of compressed expanded graphite have atotal thickness of less than 40 mm, and typically between 5 and 20 mm.24. Thermal insulation structure according to claim 19 in which the saiddense layer or layers made of compressed expanded graphite have a totalthickness of less than 2 mm, and typically of between 0.5 and 1.5 mm.25. Thermal insulation element designed to be fitted on furnacesoperating in a non-oxidising atmosphere and at temperatures of more than800° C., characterised in that it comprises a thermal insulationstructure according to claim
 19. 26. Thermal insulation elementaccording to claim 25, characterised in that it forms part of the wallof the chamber of a furnace operating at temperatures of more than 800°C. and in a non-oxidising atmosphere.
 27. Thermal insulation elementaccording to claim 26, characterised in that it is in the form of abrick, such that the assembly of several of these bricks forms thesurface of the combustion chamber of the said furnace.
 28. Thermalinsulation element according to claim 26, characterised in that it is inthe form of a cylindrical wall in one or more parts making up thecombustion chamber of the said furnace.
 29. Thermal insulation elementaccording to claim 25, characterised in that its apparent surface iscovered with a dense compressed expanded graphite layer with a densityof more than 0.4 g/cm³ (400 kg/m³) typically between 0.5 and 1.6 g/cm³(500 and 1600 kg/m³).
 30. Method for manufacturing a thermal insulationstructure, characterised in that it comprises the following steps: a)making at least one “thick” sub-dense strip with a thickness of lessthan 40 mm, typically between 5 and 20 mm, by limiting the densityobtained by compression of graphite particles to small values within therange of 0.05 g/cm³ (50 kg/m³) to 0.30 g/cm³ (300 kg/m³)); b) making a“thin” dense strip with a thickness of less than 2 mm, typically between0.15 and 1.5 mm, with a density within the range of 0.5 to 1.6 g/cm³(500 to 1600 kg/m3); c) joining said two strips, typically byco-lamination, so as to form multilayer structures that comprise analternation of adjacent thick sub-dense/thin dense layers, with at leasttwo elements, said assembling being made as follows: c1) the saidsub-dense thick strip is coated with a liquid solution rich in carbon,typically a phenolic resin, a furfuryl resin or pitch; c2) almost allsolvents in the solution, if any, are then eliminated by slow drying;c3) the said dense thin strip is then added to the coated surface; c4)heat treatment of thus joined strips under a non-oxidising atmosphere ata temperature of not less than 800° C.
 31. Manufacturing methodaccording to claim 30, modified so that two dense thin strips are madein step b) and in that a sub-dense thick strip is placed, typically byco-lamination, between the said two thin strips in step c). 32.Manufacturing method according to claim 30, modified so that twosub-dense thick strips are made in step b) and in that a dense thinstrip is placed, typically with co-lamination, between the said twosub-dense thick strips in step c).
 33. Method for manufacturing a brickof a thermal insulation element, characterised in that a thermalinsulation structure is made according to the method in claim 31, and inthat the structures thus made are then cut to the required dimensions.34. Method for manufacturing a thermal insulation element, characterisedin that it comprises the following steps: a) a thermal insulationstructure is made according to the method in claim 30, the saidstructure being sufficiently flexible so that it can be wound spirallyon a cylindrical support afterwards; b) before winding, the sub-densethick layer of the structure is coated with a liquid solution rich incarbon, typically a phenolic resin, a furfuryl resin or pitch, and thenalmost all solvents in the solution, if any, are eliminated by slowdrying; c) the structure thus obtained is wound spirally on severallayers so as to obtain a cylindrical jacket with the required thickness;d) the cylindrical face of the said cylindrical jacket that is occupiedby the sub-dense layer is covered with a flexible strip made of densecompressed expanded graphite; e) the jacket is heat treated under anon-oxidising atmosphere at a temperature equal to at least thetemperatures that the thermal insulation will need to resist during use,typically 800° C., and preferably 1000° C. or more.
 35. Method formanufacturing a thermal insulation element designed to be fitted onfurnaces operating in a non-oxidising atmosphere and at temperatures ofmore than 800° C., characterised in that it comprises the followingsteps: a) a sub-dense compressed expanded graphite layer is made with adensity between 0.05 and 0.3 g/cm³ (50 and 300 kg/m³) and with athickness of less than 25 mm, b) the said strip is curved so that it isin the form of a portion of a cylinder, c) after bending, the strip isglued, d) a reinforcing layer made of a dense compressed expandedgraphite, with a density between 0.5 an 1.6 g/cm³ (500 and 1600 kg/m³),is applied directly on one or two of the faces of the curved strip, e)the assembly is then heat treated while being held in shape by agraphite conforming jig surrounding the product.